The present invention relates to a mechanical device for adjusting automatically the length of a control cable which operates a mechanical member which is subject to dimensional wear, particularly in cases in which this wear affects the operation of a fixed-length control cable. The invention is particularly suitable for adjusting cables which are used for operating motor-vehicle clutches, for which application the present invention had been conceived, developed and tested. The invention is adaptable, with little or no alteration to other applications.
As is well known, a clutch control cable comprises a flexible, tubular casing or sheath constituted by a tight metal coil which adapts easily to the irregularly-shaped compartments of motor-vehicle engines, and a thin steel cable which can slide longitudinally in both directions within it. When the control cable is fitted along a curved path and the casing is prevented from moving longitudinally relative to the inner cable, traction or thrust can be transmitted along the path.
The clutches used in the majority of motor vehicles are disc clutches and, in general, they are associated with a fly-wheel which is fixed to the crankshaft of the gearbox and are provided with a spring or springs which keep a pressure plate, fitted loosely on the shaft, tight against the outer face of the fly-wheel.
Situated between the fly-wheel and the plate is the driven disc which slides on the same gearbox shaft and which is gripped between them to transmit the rotation of the engine to the gearbox shaft.
The control cable transmits the movement of the clutch pedal to the clutch-operating lever, which disconnects the engine from the gearbox, overcoming the force of the spring or springs which keep the pressure plate, the driven disc and the fly-wheel under pressure. The clutch is disengaged to allow gear-changing to take place and then re-engaged to enable the rotation of the engine to be transmitted to the gearbox and the wheels. It is during this frequently occurring clutch re-engagement operation, when the anti-slip lining of the disc is subject to wear, with the result that, when the wear reaches a certain value, the control cable becomes tensioned such that it has a constant, fixed length and the pressure plate fails to exert the required pressure on the clutch disc. The disc starts to slip more easily and its wear increases progressively until it fails to fulfil its function of transmitting the rotation of the shaft to the gearbox, thereby causing considerable problems and inconvenience and potentially putting the vehicle and its occupants into dangerous situations.
This fault should be removed before it reaches this extreme condition either by modification of the active length of the control cable, if possible, or by replacement of the worn driven disc. In both cases additional expense and inconvenience are caused to the user.